avoid encoding errors with unicode content piped through stdio on Win…

…dows

Consider this trivial file (with a trailing LF):

    print('This is a unicode character: ≠'.encode("UTF-8"))

This command worked in cmd.exe or an MSYS terminal, and printed ≠ correctly:

    $ cat test.py | pyupgrade.exe --py38-plus -

This crashed with an encoding error:

    $ cat test.py | pyupgrade.exe --py38-plus - > reformated.py
	Traceback (most recent call last):
      File "C:\hgdev\python39-x64\lib\runpy.py", line 197, in _run_module_as_main
        return _run_code(code, main_globals, None,
      File "C:\hgdev\python39-x64\lib\runpy.py", line 87, in _run_code
        exec(code, run_globals)
      File "c:\Users\Matt\.local\bin\pyupgrade.exe\__main__.py", line 7, in <module>
      File "C:\Users\Matt\pipx\venvs\pyupgrade\lib\site-packages\pyupgrade\_main.py", line 389, in main
        ret |= _fix_file(filename, args)
      File "C:\Users\Matt\pipx\venvs\pyupgrade\lib\site-packages\pyupgrade\_main.py", line 330, in _fix_file
        print(contents_text, end='')
      File "C:\hgdev\python39-x64\lib\encodings\cp1252.py", line 19, in encode
        return codecs.charmap_encode(input,self.errors,encoding_table)[0]
    UnicodeEncodeError: 'charmap' codec can't encode character '\u2260' in position 36: character maps to <undefined>

Since bytes are read from `stdin.buffer` and decoded as UTF-8 when the input
file is '-', it makes sense to write UTF-8 bytes to `stdout.buffer`, and avoid
using the default codepage.  The use case here is wiring this up to the `hg fix`
extension, which writes content to the tool's stdin and reads it back from its
stdout to reformat files.  That shouldn't change the encoding.

A workaround using the existing code is to set `PYTHONUTF8=1` in the environment,
but that's not obvious or always easily done.  This change also has the nice side
effect of no longer changing LF input to CRLF output.  (You'd think that
`print(..., end='')` would avoid printing the EOL, but that's apparently baked
into the `TextIO` object that is `sys.stdout`, and not something the print
function can override.)

pyupgrade/_main.py

@@ -327,7 +327,7 @@ def _fix_file(filename: str, args: argparse.Namespace) -> int:
     contents_text = _fix_tokens(contents_text)
 
     if filename == '-':
-        print(contents_text, end='')
+        sys.stdout.buffer.write(contents_text.encode())
     elif contents_text != contents_text_orig:
         print(f'Rewriting {filename}', file=sys.stderr)
         with open(filename, 'w', encoding='UTF-8', newline='') as f: